There are various known ways to connect a heating element to the body of a glow plug. As shown in U.S. Pat. No. 3,749,980 issued to Baxter on July 31, 1973, the heating element of some glow plugs includes a refractory metal sheath or sleeve which is brazed to a metallic body or housing. The above brazed joint merely provides a shearing force, in a direction axial to the joint, as the only means of retaining the heating element to the glow plug body. When such glow plugs are used in a combustion chamber of a diesel-cycle internal combustion engine, the aforementioned joint is periodically subjected to very high combustion chamber gas pressures and temperatures which can cause failure of the brazed joint. Such failure eventually permits leakage of combustion chamber gas and/or results in breakage of the electrical circuit in the glow plug.
Moreover, such a heating element having a metallic outer peripheral surface is susceptible to early failure caused by corrosion and oxidation particularly when the heating element is continuously energized in a diesel-cycle internal combustion engine in order to ignite non-autoignitable fuels such as methanol, ethanol or gaseous fuels. As shown in U.S. Pat. No. 4,661,686 issued to Yokoi et al. on Apr. 28, 1987, it is known to braze a heating element having an outer peripheral surface made from a ceramic material, such as silicon nitride (Si.sub.3 Ni.sub.4). Such a ceramic heating element may be relatively more corrosion and oxidation resistant compared to a metallic outer peripheral surface. However, in addition to the aforementioned problems generally associated with brazing, it is usually more difficult to braze a heating element made of a ceramic material (in contrast to a heating element made from a metallic material) to a metallic body of a glow plug because of the dissimilarity of the materials being brazed.
As shown in U.S. Pat. No. 4,252,091 issued to Steinke on Feb. 24, 1981, and U.S. Pat. No. 4,477,717 issued to Walton on Oct. 16, 1984, another way of connecting a heating element to a glow plug body is to use an interference fit. In Steinke, a metallic sheath of a heating element is press fitted directly into a bore of a glow plug body. In Walton, an end of a metallic sheath of a heating element is crimped onto a compressible silicon washer which is then pressed fitted into a bore of a glow plug body. The above interference joints are provided with a relatively constant frictional force which, if subjected to enough combustion chamber gas pressure, can be overcome and permit uncontrolled relative movement between the heating element and the housing. Such uncontrolled relative movement can lead to leakage of combustion chamber gas and/or breakage of the electrical circuit in the glow plug.
As shown in U.S. Pat. No. 3,992,043 issued to Whitley on Nov. 16, 1976, it is known to use a ferrule and one or more nuts to connect a flareless tube to a body part or fitting. In Whitley, a tapered peripheral end of a ferrule engages a frustoconically-shaped mouth of a channel in a body part. As shown in U.S. Pat. No. 4,556,242 issued to Kowal et al. on Dec. 3, 1985, and U.S. Pat. No. 4,568,114 issued to Konrad on Feb. 4, 1986, a ferrule may also include a means for biting, digging or cutting into a flareless tube.
To the Applicant's knowledge, the teachings of Whitley, Kowal et al. or Konrad have never been applied to the problem of connecting a heating element to a glow plug body. Moreover, applying the biting, digging or cutting means of Kowal et al. or Konrad to retain a heating element to a glow plug body wherein at least the outer peripheral surface of the heating element is a ceramic material may cause cracking or unacceptable stress concentration on the heating element.
The present invention is directed to overcoming one or more of the problems as set forth above.